Rule of thumb to keep flow attached to a surface, is that the diverging angles need to be 15 degrees or less. You're going from a narrower in between the rear tires to the full width of the car after the tires (about 24" more), so they should get wider to maximize the downforce and minimize drag.

i built the diffuser out of aluminium. i basically copied what i had seen on some of my diecast toys of racing vipers, except i didn't extend the diffuser past the rear bumper as the viper engineers had. i also didn't have a complete underbody tray for the car because i ran a flexible front spoiler approx. 1" off the ground that forced a lot of the air around the car instead of under it. i figured the most turbulent air i was trying to control was mostly coming in around from the sides. i don't remember what gauge it was, but i think it was 3/16" thick or so. [fairly rigid] i built the prototype out of cardboard and then had a local fabrication shop build it. my car had a fuel cell hanging back there that was designed to break away in a catastrophic accident, so this diffuser actually attached to the fuel cell "cage" that surrounded it. it was a pretty simple piece with the leading edge approx"2 inches from the rear axle. that leading edge had a 2" lip bent upwards at 45 degrees to keep air from going above the diffuser. each vane started all the way forward on the flat surface. in other words, the vanes were the full length of the pan. the horizontal surface of the pan was basically flush with the fuel cell until the rear edge of the cell, where it started going upwards to meet flush with the rear bumber fascia. the same thing was done on the sides of the cell to meet the side edges of the bumper fascia. the vanes were then shaped so that the bottom edges were all parallel to the ground. the bottom edges were roughly 3.5" above the road surface. the whole thing just attched to the fuel cell cage with four quick release pins because it was necessary to remove it sometimes to service the cooling pumps for the transmission and rear differential which were mounted under chassis above the diffuser. also, it was necessary to remove it to load the car into the trailer on occasion.